The goal of this project is to elucidate the stereochemical basis of antibody specificity and diversity, to show how antigenic recognition leads to a given response in the immune system, and to compare structures of several different homologous domains. Mouse myeloma proteins with binding activities toward natural antigens were purified by affinity chromatography, and their Fab fragments prepared by digestion with papain. Crystals of some of the Fab fragments were obtained from ammonium sulfate solutions, and two proteins gave crystals suitable for high resolution diffraction studies. Crystalline protein-heavy atom complexes were prepared and electron density maps were calculated using standard x-ray diffraction techniques. A three-dimensional model of one protein has been constructed using Kendrew wire models and an optical comparator.